Switching regulators, like all feedback systems, must be properly "compensated" to ensure that the feedback loop will not oscillate. Compensation is particularly difficult in switching regulators because the inductors and capacitors conventionally used (in an LC filter network) to convert the switching waveform to a steady DC output voltage cause large phase shifts in the forward part of the feedback loop. Also, the "gain" of the loop is often a function of DC input voltage. This can lead to instability problems or slow loop response if input voltage varies over a wide range.
Another problem with switching regulators is slow response to sudden changes in input voltage or load current. The regulator may have high DC gain which allows it to maintain a well-regulated output under wide variations of input voltage and load current, but if these variations occur rapidly, there will be large transient shifts in output voltage while the feedback loop attempts to correct the output error. The overall slow response is dictated by the response time of the above-mentioned LC filter network and by the requirement that the AC loop gain generally needs to be less than 0 dB (unity) at 1/10th or less of the switching frequency. A switching frequency of 100 KHz therefore will require a unity gain frequency of 10 KHz or less.
Linear regulators, in contrast, may have unity gain frequencies in the range of several hundred kilohertz to several megahertz. They respond quickly to fluctuating line or load conditions. However, linear regulators are generally not as efficient as switching regulators.
To achieve better response times to input fluctuations, prior art switching regulator circuits have used what is known as "feedforward" correction. The principle of these circuits is to add an output correcting signal to the forward part of the loop. This signal is proportional to input voltage and is designed to cause a shift in output voltage equal, but opposite to that caused by the change in regulator input voltage. This correcting signal has fast response because it does not have to pass through the slow "feedback" part of the overall regulator loop. One way to accomplish this feedforward function is to make the oscillator amplitude be a function of input voltage. Because this causes the frequency to vary inversely with input voltage, oscillator charging current also needs to be a function of input voltage to maintain constant switching frequency. Various other schemes have also been devised to insert a feedforward signal. However, these feedforward schemes suffer drawbacks. Problems encountered with these other prior art schemes include: inaccurate compensation, narrow range of allowable input voltage, and jitter in switching frequency.
In view of the foregoing, it would be desirable to be able to provide feedforward output correction in a switching regulator circuit in a manner that allows accurate compensation and a broad range of allowable input voltages, and that does not create jitter in the switching frequency.